MoD+turtle shell only works against dmg that is actually mitigated, it does not do anything against dmg taking char from >threshold to 0, so those can be ignored entirely in such cases, and it’s about the base dmg - that is why circuit breakers alone cannot always save a char, because they have to be allowed to “trip” in the first place.
Enemies got increased phys dmg after v1.3, so certain stuff simply hurts more; and for some reason this guy is repeatedly standing in the arena fire spurts, which then increases dmg done by Gargabols volcano, letting char take a full shotgun volley at max impact.
Gargabol has 1k cunning, which a bunch of phys dmg, and has the TDM modifier from the DD, + “enrage”/passive dmg buff from being brought down in health right before.
Everything the char has is running out right at the second 2nd volcano is plopped down, letting them take the total full brunt of 3½ attacks at once, a direct volcano hit, flying frag, fire spurt and whatever remaining burn dot from garga’s previous hits, since this guy was running through every pool and wave almost as if on purpose.
And the instant the 2x absorb shields wore off, and triple healing procs went down (at the same time), it was just disaster town placement too.
Even at 80 res, the fire and chaos dmg from 1 volcano alone would be about 8.8k, not factoring in the high phys dmg portion, with additional spurt dmg and burn rolling, or secondary frags.
not excluding the possibility of a bug,
but by all circumstances/without a GT showing sky high armour and phys res, i could see the death being plausible because of the manner interacted there.
*tbh feels like it’s kinda ballsy to just waltz through pools and spurts like that, while not hauling ass on volcanoes, after triggering enrage 
on HC 